The present invention is suited for, but not limited to, applications in electrical switching devices, e.g., in an intelligent circuit breaker having condition monitoring capability, in which it is necessary to employ measured current waveforms obtained by a current transformer (CT) or other saturating device to analyze the condition of the system being monitored. When a short circuit takes place on a power system, the resulting fault current contains a high-rms sine wave and a DC offset, forcing the current transformers to saturate. As a result, the observed current is distorted and adversely affects the accuracy of various numerical algorithms.
U.S. Pat. No. 5,629,869, May 13, 1997, entitled "Intelligent Circuit Breaker Providing Synchronous Switching and Condition Monitoring," discloses an intelligent circuit breaker having a means for monitoring and controlling the circuit breaker to improve its reliability and reduce maintenance costs. The disclosed condition monitoring unit (CMU) is capable of monitoring a variety of parameters associated with the circuit breaker, and to reduce maintenance costs through deferred maintenance and avoidance of unplanned outages by identifying impending failures before they occur. One example of an application in which the CMU utilizes measured currents obtained through a CT relates to the monitoring of contact and nozzle wear. Since contact and nozzle wear are a function of mechanism position and current, the required inputs are phase current from the CT secondary and mechanism position. For example, seven regions or cells of the interrupters are monitored for cumulative wear, and there is a specific mathematical expression for each of these cells that relates mechanism travel and arcing current to wear. This wear, expressed in "percent of useful life," is accumulated for each cell and stored in memory, and alarm set points are used to alert operating and maintenance personnel when the cells are approaching the end of their useful life.
A problem that occurs in systems that require accurate measurements of current waveforms is that the CT employed to measure current may become saturated, particularly when the input current has a DC offset, and therefore produce a distorted output. This well known phenomenon is illustrated in FIG. 2, which is considered in greater detail below in connection with the description of preferred embodiments of the present invention. Such saturation distortion can make the measured waveform inaccurate and unreliable, and so there is a need for a method and system for removing or correcting such distortion before the waveform is used in subsequent analyses. The present invention provides a waveform reconstruction method that recovers the undistorted waveform and can be used to enhance the accuracy of those analyses that use measured currents as inputs. The method is generic because it requires neither the CT excitation curve nor the circuit impedance (relay burden).